Due to the high cost of oil and the inherent dangers associated with nuclear energy, engineers and scientists are again turning to coal as an inexpensive and readily available source of energy. Coal is burned to generate heat which is captured as steam in packaged boilers. The steam is either used to power a turbine which turns a generator and produces electricity or used in industrial process applications. Unfortunately, the burning of coal produces SO.sub.x and NO.sub.x which is extremely hazardous to the environment.
The burners used in conventional pulverized coal boilers are inefficient insofar as they use a lot of power, require expensive coal pulverization, necessitate the application of expensive wet scrubbers to reduce SO.sub.x emissions, and produce flue gases with undesirably high NO.sub.x levels. Furthermore, NO.sub.x levels are typically reduced via selective catalytic reduction by injecting ammonium or urea, but ammonia may occur and the additive solutions are expensive.
In an effort to overcome the above-mentioned disadvantages of conventional pulverized coal burners and to provide a more environmentally acceptable means for producing energy from coal, the present inventor has investigated the use of rotating fluid bed combustors as burners in coal fired boiler applications.
Early research on rotating fluid bed combustors is set forth in U.S. Pat. No. 4,039,272 (Elliott), which issued Aug. 2, 1977, and articles by C. I. Metcalfe and J. R. Howard, "Fluidisation and Gas Combustion in a Rotating Fluidised Bed", Applied Energy, Applied Science Publishers Ltd., Vol. 3, (1977), pp. 65-73, and J. Broughton and D. E. Elliott, "Heat Transfer and combustion in Centrifugal Fluidized Beds", I. Chem. E. Symposium Series No. 43, pp. 11-1 to 11-6.
All of the aforementioned rotary fluidized bed combustors are directed to combustion of coal in drums or combustor assemblies which rotate about their vertical axis so as to form, substantially vertical beds. U.S. Pat. No. 4,039,272 (Elliott) discloses an apparatus for carrying out a reaction in a fluidized bed comprising a rotatable drum with a circumferential wall which is permeable to gases. A bed of particles is supported on the circumferential wall of the rotating drum during operation and the reactants are fed into the bed. A fluidizing gas is passed through the circumferential wall of the drum. A receiver is provided to retain small particles carried from the bed by the fluidizing gas. The small particles are returned to the bed when operation ceases. A reservoir for discharging larger particles into the bed after operation has commenced may be provided.
The aforementioned references disclose only vertically disposed rotary fluid bed combustors.
U.S. Pat. No. 5,070,821 (Virr), which issued on Dec. 10, 1991, discloses a horizontally disposed rotary fluid bed combustor which discloses a rotatable drum having a circumferentially extending wall which is permeable to gases; a means for feeding a fluidizing gas through the wall into the drum; a bed of particles which, at least when the gasifier is in use, is supported on an internal face of the wall and is fluidized by the fluidizing gas; an outlet means for receiving a carbonaceous material from outside the gasifier and delivering the carbonaceous material to the drum; a means for introducing steam into the drum; a de-NO.sub.x tube having a mixing zone and a nitrogen fixing zone; a means for introducing secondary air disposed between the mixing zone and the nitrogen fixing zone; and a means for introducing tertiary air disposed between the de-NO.sub.x tube and a main boiler or furnace.
According to the Virr patent the drum, i.e., the rotating section of the combustor assembly, is rotated by a shaft and drive belt driven by a motor. The shaft includes a post, oil seals, inner shell, outer shell and bearings. The shaft is connected to a distributor plate of the rotatable housing. The coal and limestone are fed into the fluidized bed via a chute disposed within the open side of the rotating drum which faces the de-NO.sub.x tube.
All of the aforementioned rotary combustors have been hampered by the lack of a practical method of introducing the fuel and other services such as a pilot burner to the rotating bed inside the revolving drum. The conventional method for introducing coal and limestone into a rotating bed as described in U.S. Pat. No. 5,070,821 is to feed it through a chute disposed within the open side of the rotating drum, i.e., the side closest to the de-NO.sub.x tube. This arrangement wherein the fuel and limestone feeds are introduced at or near the open side of the rotating drum typically subjects the fuel to high temperatures and premature carburization. Unfortunately, rotating fluid bed gasifiers have provided a very difficult technical problem for the introduction of the fuel and limestone into a rotating drum without premature carburization or a general loss of fuel due to the fuel being carried away together with the gaseous stream. This is both inefficient and results in an increase in SO.sub.x and NO.sub.x levels in the downstream boiler due to the non-gasified or carburized fuel which exits the gasifier into the boiler.
The present inventor has designed a unique feed mechanism for introducing fuel and limestone into a rotating fluid bed gasifier which is capable of avoiding premature carburization of the fuel and the carrying away of the fuel together with the gaseous stream.
The present invention also provides many additional advantages which shall become apparent as described below.